1. Field of the Invention
The present invention relates to an improved lithium secondary battery employing a non-aqueous electrolyte and exhibiting superior capacity and charge-discharge characteristics.
2. Description of the Prior Art
In response to the trend toward miniaturization of electronic devices, it is necessary to improve the extent to which it is possible to realize higher capacities and greater safety and reliability, and various lithium secondary batteries using carbonaceous materials as the anode have been proposed. For example, methods of using graphite as anode material are disclosed by U.S. Pat. No. 4,304,825. Japanese Laid Open Patent Application Ser. No. 1982-208079. U.S. Pat. No. 4,423,125. Japanese Laid Open Patent Application No. 1983-102464 or Japanese Laid Open Patent Application Ser. No. 1992-190555. However, since graphite contains crystallites, the intercalation and deintercalation of lithium ions damage these crystallites and impair reversibility. Additionally, the high reactivity of the lithium-intercalated anode material causes the decomposition of the electrolyte, and that, in turn, causes considerable internal short circuiting. These defects make the resulting batteries inconvenient and difficult to use.
On the other hand, carbon materials having a high surface area, such as activated carbon, are disclosed by U.S. Pat. No. 4,497,883. Batteries made in accordance therewith make use of electric double layer formation based on the high surface area of the activated carbon. However, such defects as low charge-discharge efficiency and a low battery voltage are observed when secondary batteries are made using such materials as the anode.
To overcome these obstacles, the use of carbonaceous materials differing from graphite with its crystallites and carbonaceous materials such as activated carbon with their large surface area as anodes has been proposed. More concretely, it has been proposed that materials be categorized by the temperature of calcination used to produce them, and methods using calcinated organic compounds obtained by subjecting the said organic compounds to calcination temperatures of 1500.degree. C. or less as anode materials have been disclosed in Japanese Laid Open Patent Application Ser. No. 1983-93176 and Japanese Laid Open Patent Application Ser. No. 1985-235372. Moreover, the use of carbon fibers obtained by calcination at temperatures around 2000.degree. C. as the anode is disclosed in Japanese Laid Open Patent Application Ser. No. 1985-54181, and the use of carbonaceous material having graphite structure that has been calcined at from 1000.degree. C. to 2500.degree. C. as anode material has been disclosed in Japanese Laid Open Patent Application Ser. No. 1985-221973. On the other hand, a carbonaceous material having a pseudo-graphite structure in which, according to x-ray diffraction spectroscopy, the inter layer separation distance (d.sub.002) is 3.37 .ANG. or more and the size of the crystallite c-axis (Lc.sub.002) is 150 .ANG. or less is disclosed in Japanese Laid Open Patent Application Ser. No. 1987-122066. Moreover, the use as anode material of carbonaceous material falling within the scope of a relative surface area A (m.sup.2 /g) represented by 0.1&lt;A&lt;100, and Lc.sub.002 and true density, .rho., the values for which satisfy the relations 1.70 &lt;.rho.&lt;2.18 and 10&lt;Lc.sub.002 &lt;120.rho.-189 are disclosed in Japanese Laid Open Patent Application Ser. No. 1987-90863. Additionally, Japanese Laid Open Patent Application Ser. No. 1990-66856 discloses a (d.sub.002) of 3.70 .ANG. or less, and a .rho. value of less than 1.70 (g/cm.sup.3) that moreover does not exhibit an exothermic differential thermal analysis peak at 700.degree. C. or above. Among these carbon materials, various improvements have been made and some of them have been put into practical use: however, none of them as yet exhibit adequate capacity.
Then, it was proposed that high capacity be achieved by adjusting the content of elements other than carbon present in the material to optimum levels. For example, Japanese Laid Open Patent Application Ser. Nos. 1991-137010 and 1993-74457 disclose carbonaceous materials containing the element phosphorus, and Japanese Laid Open Patent Application Ser. No. 1991-245458 discloses a carbonaceous material containing boron. Additionally, U.S. Pat. Ser. No. 5,326,658 discloses as an anode material having a high capacity, a carbonaceous material containing nitrogen made by the calcination of an organic precursor compound obtained by reacting a conjugated polycyclic compound with a nitrocompound or a nitrating agent. Japanese Laid Open Patent Application Ser. No. 1992-278751 discloses a sulfur containing carbonaceous anode material. However, adequate realization of a battery possessing a capacity that could suitably meet the demands of long term use in portable devices even by the use of carbonaceous materials listed above, was not possible, and it became necessary to find a carbonaceous material having a higher capacity.
As stated above, lithium secondary batteries made using the anode materials composed of the carbonaceous materials of the prior art do not exhibit adequate capacity. The objective of the present invention is to solve the problems of the prior art and by so doing to offer a high performance lithium secondary battery that has greater capacity, improved charge-discharge cycle characteristics and superior stability and safety.